Automatic cementing system for precisely obtaining a desired cement density

ABSTRACT

An automatic cementing system and method are disclosed for precisely controlling the density of a slurry during a continuously mixed cement application. The system includes an input water line and a dry cement hopper for supplying water and dry cement, respectively, to a mixing chamber. The mixing chamber includes two chambers, Chamber A and Chamber B, for thoroughly mixing the water and cement together to form a cement slurry. Chamber B includes a level sensor for measuring the change in slurry level. The input line includes a pump for supplying the water to the mixing chamber, and a flow meter for measuring the rate at which the water is supplied. Similarly, the hopper includes a rotary valve for regulating the rate at which the dry cement is supplied to the mixing chamber and a speed sensor for measuring the speed of the rotary valve. A discharge line with a discharge pump and a discharge flow meter receives and discharges cement slurry. A recirculation line is included for density control during initialization. A system controller receives operational parameters from the operator, including desired slurry density, mix water density and dry cement density. Once the rate of the input pump is set, the controller monitors the output from the level sensor in Chamber B and adjusts the rotary valve until the level of slurry in Chamber B is constant. At that time,, the amount of water leaving the system as slurry is calculated by the controller, and the controller then sets the rate of the input pump based upon this calculation.

BACKGROUND OF THE INVENTION

This is a divisional continuing application of copending applicationU.S. Ser. No. 08/178,659, filed Jan. 7, 1994, which is acontinuation-in-part of U.S. Ser. No. 07/969,944, filed Oct. 30, 1992(now abandoned), which was a divisional of then U.S. Ser. No.07/389,923, filed Aug. 2, 1989 (now U.S. Pat. No. 5,281,023).

The invention disclosed herein relates generally to an apparatus andmethod for obtaining a slurry with a desired density during acontinuous-mixing operation. More particularly, the invention relates toan automated system for automatically controlling the density of acement slurry with a very high degree of precision for use in wellcompletions.

Systems for mixing dry cement with water are well known in the art.Typically, cement mixing for large jobs is either done by batch-mixingor by continuous-mixing. A batch-mixing operation occurs when theingredients are mixed in a large tank or blender to obtain the entirevolume of cement slurry before the start of the job. A continuous-mixingjob, conversely, is an operation in which the ingredients arecontinuously mixed during the course of the job to produce a slurry forimmediate use. The advantages of batch-mixing cement is that the densitycan be controlled very accurately. The disadvantage is that batch-mixingmay prove to be impossible or impractical on large jobs in which a largevolume of cement slurry must be generated. Because the slurry ispremixed in a batch job, a blender or tank must be provided that islarge enough to hold all of the slurry to be used on that particularjob. Continuous mixing alleviates this problem, in that the slurry ismixed "on the fly" in a relatively small mixing chamber and is usedimmediately.

One problem, however, with continuously mixing the slurry is that it isvery difficult to control the density of the slurry with any decree ofprecision because ingredients are constantly being added and slurry isconstantly being discharged. As a result, it is common to havefluctuations in slurry density during continuous-mix operations. Incertain applications, cement density fluctuation can cause severeproblems. One example where cement density fluctuations are particularlyundesirable is in cementing operations for casing a wellbore. Thedensity of cement is especially critical for such cementing operations.

Cement is used in wells to secure casing in place in a wellbore to"complete" the well. The purpose of the cement is to seal and blockvarious zones between the casing and the wellbore. Special additives maybe mixed with the cement to alter specific properties of the cement, asrequired by the wellbore and casing characteristics and relationships. Ageneral overview of cementing casing may be found in Skinner, D. R.,Introduction to Petroleum Production, Volume I, Chapter 4: WellCompletion (Gulf Publishing Co. 1981), and in Moore, Preston L.,Drilling Practices Manual, Chapter 16 (PennWell Publishing Co. 1974).

Several terms commonly are used in cementing operations, as follows:

Cement Slurry refers to the mixture of dry or powdered cement and waterthat is injected or pumped into the wellbore;

Slurry volume refers to the volume of slurry that is obtained when agiven volume of dry cement is mixed with a given volume of water; and

Slurry density refers to the amount of dry cement per volume of water,and typically is measured in terms of pounds per gallon (also referredto as "PPG");

Different cements cure in different ways; for example, some cementsexpand as they cure, while others shrink. During the curing process,cement generally increases in temperature. Some cement mixtures willcrack or become increasingly permeable as a result of this increase intemperature during the curing process. Because the heat of the earthincreases at greater depths, cracking of the cement becomes morepronounced as the depth of the wellbore increases if cement is useddownhole in wells.

The cement and water typically are mixed on site during a cementingoperation because most wells are located in remote locations where it isimpractical to use large mixing tanks. Such an application commonly isreferred to as a continuous-mixing job. The materials used in the cementare usually prepared dry and transported to the well site, where it ismixed with liquid or "mix water" and pumped into the well. Various dryor liquid additives also may be added to either the mix water or to thedry cement as desired to alter the properties of the cement slurry. Thecement slurry normally is pumped in liquid form into a wellbore bypumping the slurry down the interior of the casing and forcing theslurry to flow from the bottom of the casing back upward between thecasing and the wellbore. After the cement has been pumped into thewellbore, it must be allowed to cure for a certain period of time thatcan vary between 12-72 hours.

By evaluating the wellbore and formation characteristics, a personskilled in the art can determine with a good deal of precision thepreferred cement density to use during the cement job to mosteffectively protect the casing and separate producing formations. If acement slurry density could be maintained within a tight tolerance of±0.1 lbs/gallon (PPG) of the preferred density, the probability of asuccessful cementing operation would be much higher. Some authors havestated that proper mixing of the cement slurry is critical to successfulcompletion of a cementing job on a well, and have proposed systems toalleviate this problem with density control in continuous-mixingoperations. See e.g. Galiana, et. al., "Cement Mixing: BetterUnderstanding and New Hardware," Oilfield Review, (April 1991); Hitt,et. al., "Process Control of Slurry Density: Impact on Field Performanceof Slurry Density," presented at the Society of Petroleum Engineers'Production Operations Symposium held in Oklahoma City, Okla., Apr. 7-9,1991; O'Neill, et. al., "New Slurry Mixer Improves Density Control inCementing Operations," presented at the Society of Petroleum Engineers'Latin America Petroleum Engineering Conference held in Rio de Janeiro,Oct. 14-19, 1990; Wienck, et. al., "Automatic Control of Bulk CementTank Levels," presented at the 24th Annual Offshore TechnologyConference in Houston, Tex., May 4-7 1992; and Stegemoeller, et. al.,"Automatic Density Control and High Specific Mixing Energy DeliverConsistent High-Quality Cement Slurries," presented at the 24th AnnualOffshore Technology Conference in Houston, Tex., May 4-7, 1992.

Unfortunately, the prior art continuous-mix cementing systems have beenunable to guarantee the density of the cement slurry within anacceptable tolerance level. Most prior art cementing systems are subjectto a wide fluctuation in cement slurry density. See the discussion inGaliana, et. al., "Cement Mixing: Better Understanding and NewHardware," Oilfield Review, (April 1991). Even the systems developedmore recently have encountered difficulty in obtaining a slurry densitywithin ±0.1 lbs/gallon. Id. One of the reasons for this variance is thatthe meters and valves used in the mixing and density control systemstypically are designed to be within a predetermined accuracy.Consequently, a certain amount of error is common in most meters. Thisis especially true with respect to the dry cement delivery system.Compounding this problem is the fact that many density control systemsattempt to obtain a desired density by fixing the amount of dry cementto be delivered, while adjusting the rate at which water is input basedupon feedback from a density sensor. See e.g. Stegemoeller, et. al."Automatic Density Control and High Specific Mixing Energy DeliverConsistent High-Quality Cement Slurries," presented at the 24th AnnualOffshore Technology Conference in Houston, Tex., May 4-7, 1992. Themeters and valves associated with the dry cement delivery system have arelatively large error associated with them that makes any controlsystem suspect that is based upon setting the delivery of dry cement toa fixed level. This error results from a number of factors such as thetendency of the dry cement to coagulate. Because of the difficulty inhandling and supplying dry cement during a cementing operation, it isvery difficult to maintain a constant slurry density.

Because of the inherent inaccuracy in all of the meters and valvestypically used in an automatic density system, and especially thoserelated to the dry cement delivery system, it is extremely difficult todesign a system that can very accurately and precisely control thedensity of a cement slurry. It is an object of the present invention toautomatically control the density of the cement slurry obtained in acontinuous-mixing application to within ±0.1 lbs/gallon (PPG) of thedesired density for cementing operations that is relatively independentof the error inherent in the meters and valves used in the system andespecially those related to the dry cement delivery system.

SUMMARY OF THE INVENTION

The present invention comprises a system and method for automaticallyand precisely controlling the density of a cement slurry, whichpreferably includes a fresh water input device, such as a pump or avalve, and a dry cement delivery system, supplying mix water and cement,respectively, to a mixing chamber. The mixing chamber combines the mixwater and cement to obtain a cement slurry that is discharged through adischarge line to a well or other desired location. The rate and amountof slurry being discharged is measured by a discharge flow meter. Theinput mix water line includes a flow meter and the dry cement deliverysystem includes a valve or other device for regulating the amount of drycement delivered to the mixing chamber.

The system controller also preferably receives as inputs from anoperator (1) the desired or target slurry density in pounds per gallon(PPG); (2) the density of the input mix water in specific gravity units;and (3) the absolute density of the dry cement, in specific gravityunits.

The discharge line of the system preferably includes a discharge valveand a discharge flow meter. The discharge flow meter provides anindication of the rate at which slurry is being discharged. With thisinformation, and the information provided by the operator, thecontroller determines the percent by volume of dry cement in the cementslurry. In other words, the controller determines how much dry cement isleaving the system as part of the cement slurry. Once the percent byvolume of dry cement is known, the controller then determines thepercent by volume of liquid in the cement slurry. After the controllerdetermines the percent by volume of liquid in the slurry, the controllercalculates the amount of liquid that must be supplied to the systemthrough the input line by multiplying the discharge rate by the percentby volume of liquid in the slurry. In this manner, the controllerinsures that the same amount of liquid is flowing into the system as isflowing out as a component of the cement slurry. Once the controllerdetermines the proper input rate of liquid into the system, thecontroller adjusts the input pump or input valve to provide the mixwater at the required rate. The input flow meter is used as feedback tobring the input water device to the proper speed or position.

After the input device is set at the proper speed or position, thecontroller sets an approximate speed or position for the valve tocontrol the delivery of dry cement. The controller uses the percent byvolume of cement in the slurry and the discharge rate, together withother factors, to determine the desired quantity of dry cement todeliver. The controller then compares the desired quantity of dry cementto be delivered with the measured valve and modifies the valve asnecessary to modify the delivery of dry cement.

The mixing chamber preferably comprises a divided tub, with Chambers Aand B, for receiving water and dry cement, with the discharge lineconnected to the chamber B. In addition, a tub level sensor is providedto monitor the change in slurry level in chamber B. After setting theapproximate speed of the dry cement valve, the controller adjusts thespeed or position of the valve to precisely control the amount of drycement supplied to the mixing chamber, based upon the level sensor. Thecontroller continues to adjust the speed or position of the valve untilthe level of slurry in chamber B remains constant.

The invention also preferably includes a recirculation line to permitthe density of the slurry to be brought to the desired density before itis discharged, and before automatic operation begins. The recirculationline includes a density meter for measuring the density of the cementslurry and for providing an indication of that density to the systemcontroller.

These and other advantages and details of the invention will becomeapparent from a review of the Detailed Description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiment of the invention,reference will be made now to the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of the configuration of the AutomaticCementing System of the preferred embodiment.

FIGS. 2A-2F illustrates a flow chart of the operation of the ACS systemof FIG. 1.

FIG. 3 shows a flow chart of the dry feed interrupt for controlling thevalve of the dry cement delivery system of FIG. 1.

FIG. 4 shows a flow chart of the mix water interrupt for controlling therate at which fluid is supplied to the mixing chamber of FIG. 1.

FIG. 5 is a flow chart showing how the rate and total volume of the mixwater is generated.

FIG. 6 is a flow chart showing how the rate and total volume of slurrydischarged is generated.

FIG. 7 is a flow chart showing how the rate of the dry cement isgenerated.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, the Automatic Cementing System 10 constructedin accordance with the preferred embodiment generally comprises an input(or "mix") water supply line 30, a dry cement source 40 for supplyingdry cement, a mixing chamber 60 for mixing dry cement and water, acement slurry discharge line 80, a recirculating slurry line 90, and asystem controller 100 for receiving output signals from the varioussystem components and for providing control signals to regulate theoperation of the ACS system.

The input water supply line 30 preferably provides mix water (or otherbase liquid) to the mixing chamber 60 for mixing with the dry cementfrom source 40. The water supply line 30 preferably includes a freshwater pump 35, an input flow meter 37, and a mixing bowl 55 forreceiving dry cement from hopper 51. Alternatively, a centrifugal pump(not shown) may be used with an input valve instead of a positivedisplacement pump. Similarly, mixing of the cement and water can takeplace in a jet mixer instead of a mixing bowl 55. The water/cementmixture preferably flows from mixing bowl 55 (or jet mixer) into mixingchamber 60.

In the preferred embodiment, the pump 35 comprises a positivedisplacement pump, while the input flow meter 37 comprises mag-type flowmeter. The pump 35 (or input valve, in the alternative embodiment)preferably connects electrically to the system controller 100 forreceiving control signals. The input flow meter 37 preferably connectselectrically to the system controller 100 and provides an electricalsignal to the controller 100 that is indicative of the flow rate of mixwater into the mixing chamber 60. Preferably, the input flow meter hasan accuracy of 0.50%. The water supply line 30 preferably connects to afresh water supply (not shown), which may be mounted on a truck, orwhich may comprise a water tower or other available water source capableof generating the required volume at the requisite rate.

The apparatus for providing dry or powdered cement to the mixing chamber60 preferably comprises a hopper 51 that includes a supply conduit 41, aweight sensor 44, a valve 45 in combination with a valve sensor 47, anda vibrator motor 49. As one skilled in the art will immediately realize,many other delivery systems may be used without departing from theprinciples of this invention. For example, an air slide may be usedinstead of the vibrator to insure uninterrupted flow of cement into thevalve 45. The hopper 51 receives the dry cement

through conduit 41, which connects the hopper 51 to a source of drycement (not shown). In accordance with conventional techniques, dryadditives may be combined with dry cement either before or during thecement operation. In addition, to insure a good flow of dry cementthrough conduit 41, the dry cement preferably is blown through conduit41. The conduit 41 preferably includes a cement inlet control valve 42that connects electrically to the system controller 100 and whichreceives control signals from the controller 100. In response to thecontrol signals from controller 100, the inlet control valve ispositioned to regulate the amount of dry cement supplied to the hopper51. As noted, the hopper 51 also preferably includes a weight sensor 44for determining the amount of cement in the hopper 51 by weight. Weightsensor 44, which preferably comprises a load cell, also connectselectrically to the system controller 100 for providing an electricalsignal to the controller 100 that represents the weight, and thus theamount, of dry cement in hopper 51.

Valve 45, or other suitable regulating device, controls the amount ofcement that is delivered to mixing bowl 55. In accordance withconventional techniques, the dry cement is dropped on top of the waterbeing injected through input line 30, at mixing bowl 55. Although notrequired in the preferred embodiment, a jet nozzle (not shown) may beprovided on the input line 30, just upstream from the mixing bowl 55.The cement/water mixture then flows into the mixing chamber 60 wherefurther mixing occurs. Other alternative mixing, arrangements cab beused, however, without departing from the principles of the presentinvention.

In the preferred embodiment, the valve 45 comprises a rotary or "starwheel" valve, with a controller 46 that can be externally regulated. Thecontroller 46 of valve 45 preferably connects electrically to the systemcontroller 100 for receiving electrical signals that are used toregulate the speed (or position if the valve is not a rotary valve) ofvalve 45 in order to control the rate at which dry cement is deliveredto the mixing chamber 60. Sensor 47 preferably comprises a star wheelspeed sensor for measuring the speed of rotary valve 45. In addition,sensor 47 preferably connects electrically to the system controller 100to provide an electrical signal to system controller 100 that isindicative of the speed (or position) of valve 45. As will be understoodby one skilled in the art, various other valves, controllers and sensorscould be used without departing from the principles of the presentinvention. For example, a slide gate may be used in place of a rotaryvalve. The position of the slide gate then would be monitored todetermine the size of the opening through which cement could pass.

The mixing chamber 60 receives mix water from the input supply line 30and dry or powdered cement from the dry cement hopper 51, and mixes thewater and dry cement to obtain a cement slurry. Other chemicals oradditives also may be supplied to the mixing chamber 60, as desired.Mixing chamber 60 preferably includes two divided chambers, Chamber Aand Chamber B, to define a weir divider for removing entrained airaccording to well known principles. Both Chamber A and Chamber B includea mixer for mixing the cement and water.

The mixing chamber 60 preferably is constructed so that the cement andwater pour into Chamber A when it is first mixed. As Chamber A becomesfull, the slurry pours over into Chamber B. The discharge line 80preferably connects to the lower side of Chamber B. In the preferredembodiment, a tub level sensor 65 is provided for measuring the slurrylevel of Chamber B. This tub level sensor 65 preferably comprises asonic level sensor, and Chamber B is constructed so that it isrelatively deep, while having a relatively small cross-section area.Obviously, other dimensions could also be used without departing fromthe principles of the present invention. By constructing Chamber B witha relatively small cross-section, changes in slurry level in Chamber Bwill be more pronounced, and thus will be more easily detected by thelevel sensor 65.

The discharge line 80 preferably receives cement slurry from the lowerside of Chamber B and supplies the slurry to the well site. Thedischarge line 80 includes a discharge flow meter 87 for measuring therate at which slurry is discharged, a manually controlled discharge pump82, and a discharge valve 85. In the preferred embodiment, the dischargeflow meter 87 comprises a non-intrusive sensor, such as a magnetic flowmeter, capable of an accuracy within 0.5%. The discharge flow meter 87preferably connects electrically to the system controller 100 to providean output signal that is indicative of the flow rate of cement slurrythrough discharge line 80. The discharge pump 82 preferably comprises acentrifugal pump that is manually controlled by the operator. One ormore triplex pumps 89 also may be provided downstream in the slurrydischarge line 80 in accordance with conventional techniques. Thetriplex pumps are manually regulated by the operator and normally arerun at full speed.

The discharge valve 85 works in conjunction with recirculation line 90.When discharge valve 85 is closed, cement slurry is forced to flowthrough the recirculation line 90. The recirculation line 90 preferablyconnects between the discharge line 80 and the mixing bowl 55. Therecirculation 90 preferably performs two separate functions. First, therecirculation line 90 enables the system to bring the cement slurry tothe desired density before discharging begins. Second, during the timecement is being discharged, the amount of slurry recirculated isregulated by the recirculation valve 97. This recirculated slurry addsenergy to the mixing process in the mixing bowl 55 and tends to dampenslurry density fluctuations in the mixing chamber 60. The recirculationvalve 97 is a throttling type valve and regulates the amount of slurryrecirculated. It is used in conjunction with pump 82 speed control tocontrol the discharge pressure in line 80. In the preferred embodiment,a nuclear density meter 95, which preferably has an accuracy within ±0.1PPG for precisely measuring the density of the cement slurry, ispreferably located immediately downstream from pump 82 and before therecirculation line connection.

Alternatively, one skilled in the art will understand that the presentinvention can be used without a recirculation line or alternatively, nodensity meter need be included. If no density meter is included, thenthe density of the cement slurry may still be obtained through the useof the input flow meter and discharge flow meter, as disclosed in parentapplication, U.S. Ser. No. 07/969,944, the teachings of which areincorporated herein. Alternatively, the density of the slurry may bemeasured by taking sample readings.

The system controller 100 preferably comprises a microprocessor-basedunit for orchestrating the operation of the ACS system. In the preferredembodiment, Motorola's 68HC11 is used as the microprocessor. Thecontroller 100 preferably includes an input, such as a keyboard oroperator panel, through which system parameters can be input by anoperator. The controller 100 also preferably includes an output fordisplaying to the operator certain critical system parameters.Preferably, the input and output of the controller 100 are designed sothat the operator is guided by a menu driven display to insure that allnecessary information is provided by the operator.

Also in the preferred embodiment, the system controller 100 may includeone or more read-only memory (ROM) units for storing look-up tables thatare loaded into the controller 100 prior to the start of a cement job.These look-up values are used to correlate step rates with error valuesfor both the mix water rate and the dry cement delivery rate. Thus, forexample, if the desired mix water rate is different than the actual mixwater rate, as measured by input flow meter 37, an error value iscalculated by controller 100 based upon the difference. The controller100 then accesses the look-up table, which preferably is stored on ROM,to determine the appropriate step rate by which the mix water rateshould be changed. The step rate, therefore, determines how quickly therate increases or decreases. A similar look-up table also preferably isused for the dry, cement delivery system to determine the step rate ofthe dry cement. In an alternative embodiment, another look-up table maybe provided that correlates the delivery characteristics of the hopperfor a given speed (or position) of the valve 45. Thus, by accessing thislook-up table, the system controller 100 can approximately determine thequantity of dry cement that will be delivered if the valve operates at agiven speed or is set at a particular position. This feature isespecially useful if an automatic start-up is desired.

The system controller 100 preferably connects electrically to: (a) theinput flow meter 37; (b) the star wheel speed sensor 47; (c) the soniclevel sensor 65; (d) the density meter 95; and (e) the discharge flowmeter 87, for receiving electrical output signals from each of thesesensors. These signals from each of the sensors are processed asdescribed more fully below to permit automatic operation of the ACS.

The system controller 100 also preferably connects electrically to: (a)the fresh water device 35; and (b) the valve controller 46, forproviding output control signals to each of these components. Thecontroller automatically adjusts the speed or position of the freshwater device 35 and the valve 45 to fine-tune the density of the slurry,as will be described more fully below.

I. OPERATION OF SYSTEM CONTROLLER

Referring now to FIGS. 2A-2F, the operation of the system controller 100will be discussed in accordance with the preferred embodiment. In thepreferred embodiment, the system controller provides a menu display tothe operator to guide the operator through both the main menu andthrough each step of the options. As shown in FIG. 2, the systemcontroller initially provides three options, as part of a main menu, tothe operator: (1) enter job data into the ACS (Job Entry Data); (2)calibrate and set-up the ACS (Cal & Setup); or (3) run the ACS (Run).The source code for the preferred programming of the controller isattached in the Appendix, incorporated by reference.

1. Entering Job Data

If the operator selects the Job Entry Data selection on the main menu,the controller will ask in step 202 whether the operator wishes to clearall stage data, or whether the operator wishes to edit existing data. Itshould be understood that the operation of the ACS could include anumber of stages, with a stage being defined as any change in operatingconditions, including a change in the cement slurry's target density, achange in the density of the input fluid, or a change in the density ortype of dry cement that is used. Thus, for example, if it is desired tochange from a target slurry of 16.0 PPG to 16.3 PPG, a different stagewould be involved. Similarly, if it was desired to add liquid additivesto the input water after operations had begun, thus changing the densityof the input liquid, a different stage would be defined. Changing fromone stage to the next could be preprogrammed into the system controllerby, for example, specifying the volume (either input or discharge) atwhich the stage was to change, or could be done manually by having theoperator designate a change in stage by activating a switch. In eitherinstance, the stage parameters could be preprogrammed into thecontroller.

The first display that will appear to the operator after the stages arecleared, or if the edit option is chosen, is a request, in step 204, toinput the desired or target density, preferably in pounds per gallon(PPG), for the cement slurry for that particular stage. As one skilledin the art will realize, other measurement units may be used withoutdeparting from the principles of this invention. In the preferredembodiment, an increment and decrement switch is used to set the desiredsetting. Obviously, other input procedures could be used withoutdeparting from the scope of the present invention. After the targetdensity for the cement slurry is entered for a particular stage, thecontroller prompts the operator in step 206 to enter the specificgravity of the input fluid for that particular stage, again through theuse of the increment and decrement switches. The controller nextrequests information regarding the dry cement specific gravity for thatparticular stage in step 208.

Once these three items are entered by the operator, the controller instep 210 asks the operator if there are other stages to be entered atthis time. If there are, then the controller cycles back through themenus to obtain the necessary information. When all stages have beenentered, the controller permits the operator to review the informationentered, by stage, and make any necessary changes. When review iscomplete, the controller returns to the main menu which again isdisplayed to the operator.

2. Calibrating and Setting Up the System

If the operator selects the Calibrate and Setup option, then thecontroller will first ask the operator in step 215 if he or she wishesto change the operating parameters or to calibrate the input mix waterflow meter. For example, if the input flow meter 37 provides outputpulses to signify to the controller 100 that 10 gallons/minute haveflowed through the meter, the operator could recalibrate the controllerto interpret the 100 pulses from the input flow meter to mean that 10.5gallons/minute have flowed through the meter. The operator, therefore,can adjust the k factor of the input meter. Similarly, in step 217, theoperator also can change the k factor of the discharge meter. Thisfeature enables the various sensors to be pretested and finely tuned foraccuracy. In addition, if a new meter is substituted, the meter can bequickly and accurately calibrated through the use of this softwareoption.

In step 219, the controller asks the operator if he or she wishes tochange the settings of the valve on the dry cement source. Threedifferent parameters can be changed on the valve. The first parameterthat can be changed is the response factor during normal run operations.The response factor represents the gain or amount of change that is usedwhen the dry cement rate must be changed. If a discrepancy existsbetween the actual and desired rate of which dry cement is supplied, theactual race must be changed by a certain amount. If the amount of changeis excessive and the system is over-correcting, oscillation could occur.Consequently, a gain correction is used which is only a percentage ofthe difference between desired and actual rates. The second parameterthat can be altered is the k factor that represents the interactionbetween the controller and the k factor permits calibration of thesensor. Lastly, the start-up gain of the valve can be adjusted. Thestart-up gain is the response factor of the valve during start-upoperations, which is similar to the response factor during normaloperation. All of these factors can be adjusted to fine tune theoperation of the dry cement valve.

The controller also asks the operator if it is necessary to adjust thedensity meter in step 221. The operator can adjust the controller sothat the maximum output from the density meter represents a specificdensity value. Similarly, the operator can recalibrate the controller sothat the lowest output from the density meter represents a zero densityvalue. Thus, for example, if the density meter provides an output thatranges between 0.01 to 0.20 amperes, the controller would correlate the0.01 amps to a zero density, and would correlate the 0.20 amps to themaximum density value, which might be, for example, 20.0 PPG.

Finally, the controller asks the operator if the mix water responseshould be readjusted in step 223. The mix water response, like theresponse factor of the dry cement rate, regulates the gain by which themix water is adjusted, to prevent over-correction.

3. Run Mode

If the operator selects the Run option off of the Main Menu, the systembegins automatic operation. In the preferred embodiment, the Run mode isselected after initialization and startup of the system.

In step 301, the controller calculates the current stare and runningparameters, and then displays a screen (not shown) for the operator. Thedisplay screen preferably includes mix water rate, discharge rate,target density, actual density measured by the density meter 95, thestage, the total discharge, the stage discharge, and the time.

After the screen is displayed (or updated), the Controller 100determines in step 303 if the operator has requested that the systemproceed to the next stage. This preferably is done by depressing apreselected switch for a predetermined minimum time. Alternatively, thestage may be changed by a key board entry, or automatically is apredetermined volume of slurry has been discharged.

In step 305, the Controller 100 determines whether the PPG trim featurehas been activated and, if activated, determines the amount of trimselected. Next, in step 307, the Controller reads the tub level measuredby tub level sensor 65 and scales that measurement to a physical unit(such as feet or inches) for future reference. Similarly, in step 309,the controller reads the density measured by density meter 95 and scalesthat measurement to a physical unit (such as pounds per gallon or PPG).

In step 310, the controller determines if the automatic densitycorrection feature has been enabled by the operator. In the preferredembodiment, a switch is provided that permits the operator toautomatically correct the density through the use of the density meter95 as a feedback device. In normal operation, the controller maintainsthe level in slurry in Chamber B constant, and then determines theamount of mix water and dry cement that is being discharged as slurry.These numbers are used to automatically regulate the rate of mix waterand dry cement being input to the mixing chamber. In addition to thisnormal operation, the ACS also has the capability to monitor the actualdensity of the slurry by means of the density meter 95. According to thepreferred embodiment, the operator must select this feature.

If the automatic density correction feature is selected, the controller,as shown in step 311, determines if the actual density measured by thedensity meter 95 is above or below the target density. If the actualdensity is higher than the target density, the target density isautomatically decreased, which will result in the actual density beingdecreased as ratios of mix water and dry cement are changed.

Conversely, if the measured density is low, the target density isautomatically increased, thus causing the actual density to increase asthe ratio of mix water and dry cement change. According to the preferredembodiment, the target density is only increased or decreased each cycleby five percent (5%) of the difference between actual density and targetdensity. This is done to prevent overcompensation and wide fluctuations.

If the automatic density correction is not selected, or if after it iscomplete, the controller in step 312 determines what the actual mixwater rate is into the mixing chamber. To determine the mix water rate,the controller proceeds to the low level interrupt shown in FIG. 5 togather the period for measurement and the amount of flow. FIG. 5 is asubroutine that measures pulses off of the input flow meter 37 andgenerates a period and a total. The controller, in step 312 uses thetotal number of pulses for the period, and determines the amount of flowby referring to the k factor obtained during calibration and set-up forthe mix water flow meter. Once the amount of flow and period areascertained by the controller, the flow is divided by the period toobtain the mix water rate.

In similar fashion in step 313, the controller determines the dischargerate. The controller proceeds to the low level interrupt shown in FIG.6. FIG. 6 is a subroutine that measures pulses off of the discharge flowmeter 87 and generates a period and a total flow. The controller usesthe total number of pulses generated by the discharge flow meter forthat period, together with the k factor obtained during calibration andset-up for the discharge flow meter, to determine the amount of slurrydischarged during the period. The controller then divides the amount ofslurry discharged, by the period, to determine the discharge rate.

In step 315, the controller checks to determine if the discharge ratehas changed by more than one-half of a barrel in a second. If such achange is detected, the controller reverts to the start-up mode. Next,in step 317, the controller determines if the discharge rate has passedbelow a minimum threshold. If this occurs, the ACS is shut down.

In step 319, the controller determines the percentage of volume ofcement slurry that should be mix water, and then calculates the propermix water rate. According to the preferred embodiment, the controllerdetermines the percentage of mix water (or fluid) by volume in thecement slurry. This is done by first determining the percentage of drycement by volume of slurry as follows:

    % VOLUME OF CEMENT=(SLURRY SG-FLUID SG)/(CEMENT SG-FLUID SG), (1)

wherein

SLURRY SG represents the target or desired density of slurry in specificgravity units. This means the target density, which according to thepreferred embodiment is entered in pound per gallon (PPG) by theoperator in step 204, must be convened by controller into specificgravity units;

FLUID SG represents the density of the input fluid in specific gravityunits, as entered by the operator in step 206. Where water is used asthe input fluid, this number is 1.0; and

CEMENT SG represents the absolute density of the dry cement in specificgravity units, as entered by the operator in step 208.

Once the percentage volume of cement (% VOLUME OF CEMENT) is calculated,the percentage volume of liquid may be easily calculated, as follows:

    % VOLUME OF LIQUID=1.00-% VOLUME OF CEMENT.                (2)

After the controller determines the % VOLUME OF LIQUID, the controllermultiplies this number by the DISCHARGE FLOW RATE to determine theoptimal rate at which the input fluid (or mix water) should be suppliedto the mixing chamber.

In step 321, after the optimal mix water rate is determined in step 320,this optimal rate is compared with the mix water rate measured in step312 to determine if the measured mix water rate is less than the optimalrate. If the measured rate is less than the optimal or desired rate, thecontroller determines if the ACS is in a start-up mode in step 322.Depending on whether the ACS is in a start-up mode, the controlleraccesses one of two look-up tables--one for start-up mode and one fornormal operation. Each of the look-up tables defines a step rateassociated with the particular magnitude of error between the measuredmix water rate and the desired or optimal mix water rate. Each magnitudeof error is associated with a particular step rate in the look-up table.The step rate represents how quickly the mix water rate will beincreased or decreased.

For example, if the controller determines in step 321 that the measuredmix water rate is 5 gallons per minute less than the optimal rate, thecontroller, in step 323 or 324 (depending on whether the ACS is innormal operation or start-up), will access a look-up table, determinewhat step rate is associated with a discrepancy or error of 5 gallonsper minute. If the error or discrepancy is very small, the step rate maybe zero to reflect no change and to prevent overcompensation. In thepreferred embodiment, the look-up is preprogrammed into read only memory("ROM").

After the controller determines the appropriate step rate is either step323 or 324, a direction flag is activated to remind the controller thatthe actual mix water rate must be increased. Conversely, in steps 325,326, and 327, if the mix water rate is too high, the controllerdetermines the magnitude of the error or discrepancy between measuredmix water rate and optimal mix water rate, and determines from theappropriate look-up table what the proper step rate is. After the steprate is determined, the direction flag is activated down to remind thecontroller that the actual mix water rate must be decreased by the stepfactor.

Because a separate look-up table is used for the start-up mode and fornormal operation, the reaction of the system can be fine-tuned to permitquicker (or slower) response for start-up than would be desired fornormal operation. After the appropriate step rate and directionalinformation has been found, it can be stored in step 329. In thepreferred embodiment, the information is stored by means of pulse widthmodulation.

The stored step rate and directional information for mix water rate isretained by the interrupt task shown in FIG. 4. In the preferredembodiment, the interrupt of FIG. 4 occurs at a 0.040 second rate. Themix water interrupt determines in step 251 the step rate and directionalinformation. In response, the controller increases or decreases thespeed or position of the input water device 35 in accordance with thestep rate and directional information.

In step 335, the controller obtains the rate at which dry cement issupplied by dry cement source 40. A sensor 47 on the dry cement source40 generates pulses that are retrieved in the low level interrupt ofFIG. 7. The controller converts the pulses into rate information bymeans of the k factor.

After determining the rate at which dry cement is being introduced intothe mixing chamber, the controller determines in step 337 if the systemis in the start-up mode and branches accordingly. Regardless of whetherthe system is in start-up, the controller performs a number ofcalculations in steps 338, 339 to determine the amount of bulk cementrequired.

The first calculation performed in steps 338, 339 is to determine theWEIGHT PER CUBIC FOOT OF MIX WATER (or "HO") as follows:

    HO=7.4805*8.3452984*WATER SG                               (3)

7.4805 represents gallons per cubic foot;

8.3452984 represents pounds per gallon; and

WATER SG is the density of the mix water or input fluid in specificgravity units, as entered by the operator in step 206.

Next, the controller calculates the % of "REAL" DRY CEMENT ("CO") in thetotal bulk volume of the dry cement, as compared to the percentage ofair in the total bulk volume of dry cement, as follows:

    CO=BULK WEIGHT OF DRY CEMENT/(CEMENT SG*HO),               (4)

Where,

BULK WEIGHT OF DRY CEMENT is the weight of dry cement in a sack ofcement;

CEMENT SG represents the absolute density of the dry cement in specificgravity units; and

HO is the weight per cubic feet of mix water found in calculation (3).

The controller then determines the ABSOLUTE VOLUME OF DRY GALLONS PERMINUTE ("DRY GALLONS MINUTE"), as follows:

    DRY GALLONS MINUTE=CUBIC FEET MINUTE DRY*7.4805            (5)

Where

CUBIC FEET MINUTE DRY represents the quantity of cubic feet of drycement added each minute.

Finally, the controller determines the desired rate of dry cement ("DRYDEMAND") to be supplied as follows:

    DRY DEMAND=DISCHARGE FLOW RATE*% VOLUME OF CEMENT          (6)

Where

DISCHARGE FLOW RATE is the measured discharge flow rate found in step313; and

% VOLUME OF CEMENT is the percentage of dry cement by volume of slurry,as calculated in step 319.

If the controller determines in step 337 that the ACS was in a start-upmode, the controller branches to path 341. After calculating the rate ofdry cement required in step 338, the controller compares this rate withthe actual rate determined in step 335. If the controller determines instep 344 that the actual rate at which dry cement is being delivered islower (or higher) than the desired rate, then the controller branches tostep 346 (or step 348) where a step rate is obtained and a directionalflag is set. The step rate preferably is found in a look-up table,associated with a particular magnitude of error. Thus, the controllerdetermines the error or discrepancy between the actual dry cement rateand the desired rate, and looks up that error in the look-up table tofind the step rate that should be used to correct the actual rate atwhich dry cement is delivered.

After the step rate is found, the current tub level is stored in memoryin step 350. If, conversely, the ACS is not in a start-up mode, then thecontroller follows branch 342. After determining the desired rate of drycement to be supplied, the controller in step 360 determines the levelin Chamber B. In step 362, the controller compares the level measured inChamber B with the level stored in memory during start-up (step 350). Ifa difference exists in tub level, the controller determines themagnitude of the difference in step 363 or 364, and then multiplies thedifference by the dry demand found in step 339 to obtain a "tub factor".The tub factor then is either added to or subtracted from the dry demandcalculated in step 339 to obtain a modified dry demand that then isused.

Next, in step 370, the controller compares the desired modified rate atwhich dry cement should be delivered with the actual rate. In steps 371and 372 the controller determines the appropriate step rate from thelook-up table. After the appropriate step rate has been found, thecontroller in step 375 shows the step rate and directional information,preferably in a pulse width modulated signal. This signal is retrievedin the Dry Feed Interrupt of FIG. 3. As shown in FIG. 3, the speed orposition of the dry cement valve is altered according to the step rateand directional rate obtained from step 371 (or step 372). In step 380,the controller determines if the tub is maintaining a constant level andsets the appropriate flags.

II. ACS OPERATION

The operation of the ACS system will now be described with reference toFIG. 1. The operation of the ACS preferably comprises an initializationmode, a start-up mode, a run mode, a trim mode, and a rate variancemode.

A. Cementing Initialization

To initiate the ACS, the operator manually (1) opens the input watervalve 36 to permit water to flow into the mixing chamber 60; and (2)adjusts the speed of the rotary valve 45 to regulate the delivery of drycement into the mixing chamber 60. Because the discharge valve 85 hasyet to be opened, the slurry will be forced to recirculate throughrecirculation line 90. As the slurry flows through recirculation line90, the density meter 95 measures the density of the slurry.Alternatively, the density can be measured manually by an operator.

Recirculation continues in this manner until two conditions occur: (1)the density measured is the same as the target or desired density, and(2) the level of cement in Chamber B of mixing Chamber 60 lies betweenthe levels marked in FIG. 1 as x_(o) and x₁. At any time prior tostarting the run mode, the operator enters into the system controller100 the following information for each operating stage: (a) the desiredor target slurry density; (b) the density of the water or other baseliquid being supplied through input line 30; and (c) the density of thedry cement being used.

B. ACS Startup

After the ACS system has been initialized, the operator opens thedischarge valve 85 to allow the cement slurry to be discharged throughdischarge line 80. At this time, the ACS controller 100 receives anoutput signal from the discharge flow meter 87 indicating that dischargeflow has begun. The operator also manually sets the discharge pumps 89located downstream in the slurry discharge line 80 to operating speed.When the discharge pumps 89 are brought up to operating speed, the levelof slurry in Chamber B begins to fall.

Simultaneously, ACS controller 100 regulates the input water rate byadjusting the fresh water positive displacement pump 35 to maintain aconstant high precision ratio of input water rate to discharge cementrate. This is done by receiving a signal that represents the rate offlow of cement slurry through the discharge flow meter 87 and bydetermining the percentage of water by volume in the cement slurry. Thecontroller 100 multiplies the discharge flow rate by the percent ofwater by volume to determine the amount and rate of water that is beingdischarged. Once this flow rate of water is determined, the controller100 adjusts the fresh water pump to this rate. Thus, the controller 100precisely controls the rate at which water is injected into the mixingchamber 60 so that it is the same as the amount of water that isdischarged from the mixing chamber 60 (as part of the slurry). As aresult, it is possible to obtain very precise control of the density ofthe slurry.

According to the preferred embodiment, the controller 100 determines thepercentage of water (or liquid) by volume in the cement slurry by firstdetermining the percentage of dry cement by volume in the slurry. Thepercentage of cement by volume of slurry is calculated as follows:

    % VOLUME OF CEMENT=(SLURRY SG-FLUID SG)/(CEMENT SG-FLUID SG),

wherein

SLURRY SG represents the target or desired density of slurry in specificgravity units. This means the target density, which according to thepreferred embodiment, is entered in pound per gallon (PPG) by theoperator in step 204, must be converted by controller into specificgravity units;

FLUID SG represents the density of the input fluid in specific gravityunits, as entered by the operator in step 206. Where water is used asthe input fluid, this number is 1.0; and

CEMENT SG represents the absolute density of the dry cement in specificgravity units, as entered by the operator in step 208.

Once the percentage volume of cement (% VOLUME OF CEMENT) is calculated,the percentage volume of liquid may be easily calculated, as follows:

    % VOLUME OF LIQUID=1.00-% VOLUME OF CEMENT.

This equation assumes that only two ingredients are combined to form theslurry--input liquid and dry cement. Additives may be provided as longas they are added to either the dry cement or to the input liquid priorto their introduction to the system.

After the controller 100 calculates the % VOLUME OF LIQUID, thecontroller 100 multiplies this number by the DISCHARGE FLOW RATE todetermine the rate at which the input fluid should be supplied to themixing chamber 60. The controller then sets the input pump at the properspeed to supply the input liquid at the requisite rate. The controllermonitors the output from the input flow meter 37 to adjust the inputpump 35 to the proper setting.

For example, if the discharge flow meter indicates that slurry is beingdischarged at 420 gallons/minute, and the controller determines from theoperator inputs that the % VOLUME OF CEMENT is 0.519, then thecontroller determines that the % VOLUME OF FLUID is 0.481. Thecontroller 100 multiplies the % VOLUME OF LIQUID by the DISCHARGE FLOWRATE, as follows: 0.481×420 gallons/minutes=202 gallons/minute of inputliquid. The controller 100 then sets the speed of the fresh water pump35 at a speed to inject 202 gallons/minute. The input flow meter 37provides feedback to the controller 100 of the actual rate of liquidbeing injected into the mixing chamber 60 to permit the controller toreadjust the speed of the fresh water pump 35 to precisely control therate at which liquid is supplied to the mixing chamber 60.

At approximately the same time that the ACS controller 100 is settingthe speed of the fresh water pump 35, it also is adjusting the speed ofthe valve 45. The ACS controller 100, as noted, determines the amount ofliquid that must be injected into the mixing chamber 60 to obtain thedesired slurry density at the fixed discharge rate. At the same time,the ACS controller 100 also calculates the rate at which the dry cementmust be delivered from the source 40 to achieve the desired slurrydensity.

C. ACS Run Mode

Ideally, if the input flow meter 37 is extremely accurate permittingprecise control of the fresh water pump 35, and if the discharge flowmeter 87 is equally accurate, and if the valve 45 is delivering cementat the proper rate, so that the amount of dry cement and water beingsupplied to the mixing chamber 60 exactly equals the amount of cementand water being discharged as slurry, then the level of cement slurry inChamber B will be fixed and stable, and the actual density of thedischarged slurry will equal the desired slurry density input by theoperator into the ACS controller 100. In other words, if all meters andvalves are completely accurate, the level of slurry in Chamber B will beconstant at this time.

If, conversely, the level of cement slurry changes or fluctuates inChamber B, then the controller assumes there has been a fluctuation inbulk density of the dry cement and modifies its bulk densitycalculation. The controller then determines how much to change thedelivery rate of the dry cement to maintain tub level and proper slurrydensity. This is explained more fully in the flow chart shown in FIG. 2,and described above. Once the proper rate of dry cement is determined,the position or speed of valve 45 is changed as required.

D. PPG Trim

After the actual and desired slurry density readings are equal, it iscommon to take a sample of the cement slurry being discharged to testit. Alternatively, another density sensor may be provided downstream inthe slurry discharge line. If the density of the sample discharge (or ofthe second density sensor) does not correspond to the density measuredby density meter 95, then the operator can elect to use the ACS PPG Trimmode by pressing an appropriate switch on the control panel (not shown).Activation of this trim switch will be transmitted to the ACS systemcontroller 100, which, in response will cause the ACS to operate in thetrim mode. The trim switch permits the operator to manually alter themix water/dry cement ratio in response to discrepancies between thetarget density and measured density.

In the trim mode, the controller 100 adjusts the ratio of input waterrate to discharge cement slurry rate, which was based upon the percentof water by volume obtained from the calculations relating to the cementslurry discharge. By changing this ratio, the rate at which water is tobe supplied to the mixing chamber 60 also will change, as will the rateat which dry cement is delivered to the mixing chamber 60. As a result,the operation of the ACS will revert to the Startup Mode after the ratioof input water rate to discharge slurry rate is changed, and thecontroller 100 will proceed to determine the proper operating speed orposition of the input device 35, based upon the new ratio. After thespeed or position of input device 35 is set, then the speed of valve 45is set to an approximate value based upon desired demand of dry cement.The controller 100 then monitors the output from the level sensor 65 andadjusts the desired dry demand rate until the slurry level in Chamber Bremains constant.

E. Discharge Rate Variance

If during ACS operation, the discharge rate changes from its fixed valuefor any reason, then ACS returns to the Start-up Mode. After detecting alarge change in the discharge rate, the controller returns to thestart-up mode, where a mix water rate is calculated. The controller 100sets the input device 35 to the necessary setting so that the amount ofwater entering through input line 30 equals the amount of water leavingas slurry through the discharge line 80. The input flow meter 37 is usedby the controller 100 as part of a closed loop feedback system toprecisely set the rate of the device 35. After the mix water input rateis set, the controller then determines the approximate speed of valve 45that will provide the same amount of dry cement as is leaving as acomponent of the slurry. The controller 100 monitors the level sensor 65and adjusts the dry cement rate as necessary to maintain the level ofslurry in Chamber B constant.

Operation of the ACS system preferably continues until the total volumeof slurry discharged equals the total volume entered by the operatorinto controller 100 at the beginning of the operation or until theoperator indicates that it is time to begin a new stage. The controller100 keeps track of total volume based upon the signals from thedischarge flow meter 87. The controller 100 preferably obtains a signalfrom the discharge flow meter 87 once every 1/x of a second. Thus, thecontroller multiplies each rate signal from the discharge flow meter 87and multiplies that rate by 1/x to obtain a number that represents thetotal volume of slurry that has been discharged during that samplingperiod. The controller keeps a running count of the discharged volume byadding each new volume to the total accumulated volume, and compares thenew total with the desired total. When the accumulated total equals oris greater than the desired volume, the controller 100 terminatesoperation and notifies the operator that the job is complete.

While a preferred embodiment of the invention has been disclosed,various modifications can be made to the preferred embodiment withoutdeparting from the principles of the present invention.

We claim:
 1. An automatic cementing system, comprising:a mixing chamberwith a first chamber for receiving liquid and dry cement and a second,relatively deeper chamber to which the first chamber discharges, saidchambers mixing the liquid and dry cement together to form a cementslurry; an input line for supplying the liquid to said first chamber;means for supplying the dry cement to said first chamber, said supplyingmeans including a means for regulating the rate at which dry cement isdelivered; a level sensor located in said second chamber for measuring achange in level in said second chamber; a discharge line connected tosaid second chamber for supplying the cement slurry; and a systemcontroller electrically connected to the regulating means on saidsupplying means, and to the level sensor, for receiving signals at veryshort predetermined intervals from said level sensor indicating a changein slurry level in said second chamber and controlling said regulatingmeans to maintain the level of slurry in the second chambersubstantially constant.
 2. A system as in claim 1, further comprising;aninput device on said input line for controlling the rate at which liquidis added to the mixing chamber; a discharge flow meter on said dischargeline electrically connected to the system controller: said systemcontroller receiving a signal from the discharge flow meter indicatingthe flow rate of cement slurry and regulating the input device to addliquid to the mixing chamber at the same rate as the liquid isdischarged in the slurry.
 3. A system as in claim 2, wherein said inputdevice is a positive displacement pump.
 4. A system as in claim 2,wherein the system controller receives a signal from said discharge flowmeter and in response regulates the operation of said input device.
 5. Asystem as in claim 4, wherein the system controller receives data inputsfrom an operator, calculates the percentage of liquid in the cementslurry based upon the data inputs and controls the operation of theinput device to maintain the ratio of the rate of the liquid deliveredto the mixing chamber to the rate of the slurry discharged from thesecond chamber substantially constant.
 6. A system as in claim 5,wherein the discharge flow meter has an accuracy of approximately 0.5percent or better.
 7. A system as in claim 6, wherein the discharge flowmeter is a non-intrusive flow meter.
 8. An automated system forcontrolling the density of a cement slurry, comprising:a mixing chamberwith a first chamber for receiving dry cement and water to produce acement slurry, and a second chamber for receiving overflow from thefirst chamber; a level sensor in said second chamber for detecting achange in slurry level in said second chamber and for providing anoutput signal indicative of a change in slurry level; an input line,including an input device, for supplying liquid to the mixing chamber ata particular rate; a means for delivering dry cement to said mixingchamber, wherein the delivery of dry cement from said delivery means iscontrolled by a valve; a system controller for receiving the outputsignal from said level sensor and for providing a control signal to saidvalve to modify the delivery of the dry cement so as to maintain thelevel of cement slurry substantially constant in said second chamber. 9.A system as in claim 8, further including;a discharge line connected tosaid second chamber for discharging the cement slurry from said secondchamber, said discharge line including a discharge flow meter providingan output signal to said system controller that is indicative of therate of slurry being discharged; wherein the setting of the input deviceand the valve is dependent upon the output signal from said dischargeflow meter.
 10. A system as in claim 9, further comprising:a densitymeter in the discharge line in electrical connection with the systemcontroller for providing a signal representative of the slurry density:wherein the system controller corrects the setting of the input deviceand the valve as a function of the signal from the density meter.
 11. Anautomatic cementing system, comprising:a mixing chamber for receivingmix water and dry cement, and for mixing the liquid and dry cementtogether to form a cement slurry; an input line with an input device forsupplying the mix water at a particular rate; means for delivering thedry cement, which includes a regulating means for controlling the rateat which the dry cement is delivered; a discharge line with a dischargepump and a discharge flow meter for measuring the rate at which slurryis discharged from the mixing chamber and for providing an output signalindicative of said rate; a system controller electrically connected tothe input device, the regulating means and the discharge flow meter, forreceiving a discharge flow rate signal from the discharge flow meter andfor providing control signals to the input device; wherein the systemcontroller provides a control signal to the input device to control therate at which mix water is supplied to the mixing chamber based upon thedischarge flow rate signal; wherein the system controller determines thepercentage of mix water in the cement slurry and the amount of mix waterbeing discharged in the cement slurry from the mixing chamber, andmodifies the input device to provide an equal amount of mix water intothe mixing chamber.
 12. A system, as in claim 11, wherein the systemcontroller also sets the regulating means based upon the discharge flowrate signal.
 13. A system as in claim 12, wherein the system controllerdetermines the percentage of dry cement in the cement slurry and theamount of dry cement being discharged in the cement slurry, and modifiesthe regulating means to provide an equal amount of dry cement into themixing chamber.
 14. A system as in claim 13, wherein the regulatingmeans comprises a rotary valve with an internal control unit.
 15. Asystem as in claim 14, further comprising a speed sensor for providingfeedback to the system controller recording the actual speed of therotary valve.
 16. A system, as in claim 11, further comprising;an inputflow meter for providing feedback to the controller indicative of inputflow rate to enable the controller to precisely set the input device.17. A system as in claim 11, wherein the delivering means comprises adry cement hopper, and the dry cement hopper includes;a weight sensorfor determining the weight in the hopper; and a cement inlet controlvalve for regulating the amount of dry cement that is blown into thehopper.
 18. A system as in claim 11, further comprising;a level sensorin the mixing chamber for monitoring a change in the level of slurry inthe mixing chamber, said level sensor electrically connected to saidsystem controller for providing an indication of a change in level tothe controller, and said controller adjusting the speed of theregulating means in response to the indication of a change in level. 19.A system as in claim 18, wherein the mixing chamber comprises a firstchamber for receiving liquid and slurry, and a second chamber to receiveany overflow from the first chamber, with the discharge line connectedto the second chamber and the level sensor being located in the secondchamber.
 20. A system as in claim 19, wherein the second chamber isrelatively deep, but has a relatively small cross-section to enable thelevel sensor to easily detect changes in slurry level in the secondchamber.
 21. A system as in claim 11, further comprising;a recirculationline connected to said discharge line for recirculating slurry to themixing chamber.
 22. A system as in claim 11, further comprising;adensity meter for measuring the density of the slurry and providing anelectrical signal to the system controller indicative of the measureddensity.
 23. A system as in claim 22, wherein the controller comparesthe density measured by said density meter with a preprogrammed desireddensity, and generates a correction factor if a discrepancy exists. 24.A system as in claim 23, wherein the controller adjusts the rate of theinput device based upon the correction factor.